Hydropower development in large tropical rivers is rapidly expanding. Despite the efforts to mitigate impacts, fisheries have consistently declined and the question remains whether large hydropower dams can be designed to maintain viable riverine fisheries.
We consider that a fundamental shift in planning and design is needed, at both catchment and site scales. Large hydropower projects follow a similar development sequence of: i) proposal and concept design, with optimised energy estimate; ii) business case; iii) detailed concept; iv) impact identification; and v) mitigation. Fish passage is frequently identified as a mitigation and is developed in the last step. The project sequence leaves designers (engineers and biologists) to compromised solutions because: the dam site is chosen; the budget is set; the dam design has mostly been done; and fixed energy estimates limit the amount of water that can be used for fish passage.
We recommend integrating impacts and mitigation in the first step before the energy estimate and business case. At the catchment scale this enables a more strategic evaluation of dam sites based on ecology and habitats, especially the spatial scale of cyclic migrations and flowing water (lotic) habitats required for spawning and larval drift. At the site scale, it enables a major rethink of fish passage. Upstream passage has a record of very poor performance at large tropical dam sites, but we consider that it is potentially solvable with research, conservative designs and early cooperation of planners, biologists and engineers. Downstream passage, however, is rarely considered and major knowledge gaps remain – particularly larval drift, and design of screens and turbines. We present an alternative to mainstem thinking on dam design, and a practical way forward for developers and fisheries managers to consider.